1. Field of the Invention
The present invention relates to a radio communication device, and more particularly to a radio communication device that is preferably used for communication with a reader-writer in an RFID (radio frequency identification) system, for example.
2. Description of the Related Art
In recent years, RFID systems have been put into practical use as commodity information management systems. In such an RFID system, communication between a reader-writer and an RFID tag (also referred to as a radio communication device) attached to a commodity is carried out in a non-contact manner for transmission of information. The reader-writer and the RFID tag each have a radiation element (an antenna) for sending and receiving radio signals. Such an RFID system is typically an HF system using high-frequency wave in a band around 13.56 MHz or a UHF system using high-frequency wave in a band around 900 MHz.
In a case of the HF system, the antennas of the RFID and the reader-writer are usually coil antennas, and these coil antennas are coupled with each other via an induction field. Hence, radio signals are transmitted through a near field.
Recently, it has been suggested that a coil antenna be incorporated in a communication terminal device such as a cellphone so that the communication terminal itself can be used as a reader-writer or an RFID tag, as described in the Japanese Patent Laid-Open Publication No. 2006-270681 and WO 2007/060792.
However, as discussed in the Japanese Patent Laid-Open Publication No. 2006-270681 and WO 2007/060792, because an HF system requires a large size of coil antenna, it is necessary to provide a large space for the antenna in the casing of the communication terminal device. Also, a liquid crystal display panel, a keyboard, an RF circuit board, a battery pack and other components are provided in the casing of the communication terminal device, and metal parts of these components obstruct generation of an induction field at the coil antenna. This can cause a change in the resonant frequency of the coil antenna, which results in a failure in ensuring a satisfactory communication distance.